1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image on a recording material which is utilized for an image forming apparatus which adopts an electrophotographic system, an electrostatic recording system or the like, such as copying machines, printers and facsimiles.
2. Description of the Related Art
A heat fixing method which heats and melts an unfixed toner image and fixes the unfixed toner image on a recording material has been commonly employed as a fixing method for the image forming apparatus from the viewpoints of safety and fixing performance.
With the fixing apparatus of the above heat fixing method, there is a problem that the temperature at a non-sheet passing portion rises when recording materials whose width is smaller (hereinafter, called small size sheets) than recording materials of the maximum sheet passing width (hereinafter, called maximum size sheets) are continuously passing.
In a case that recording materials of various sizes (widths) pass through a fixing area, a part of the fixing area through which the recording material passes is denoted as a sheet passing area, and the rest of the fixing area other than the sheet passing area is denoted as a non-sheet passing area. Further, a surface part of a heating member, such as a fixing roller surface, a fixing film surface and a pressure roller surface, which passes through the sheet passing area during rotation, is denoted as a sheet passing area passage surface. And, a surface part of the heating member which passes through the non-sheet passing area during rotation is denoted as a non-sheet passing area passage surface.
In the case that the maximum sheets are passing and fixing is performed, the temperature distribution of the heating member surface becomes approximately even along the entire fixing area. On the contrary, in the case that the small size sheets are continuously passing and fixing is performed, the temperature of the non-sheet passing area passage surface of the heating roller rises excessively. This is because heat is partly accumulated at the non-sheet passing area by the amount of heat which is not discharged to the sheets when the small size sheets are continuously passed.
In general, the non-sheet passing portion temperature rise becomes large under the condition that heat discharge to the sheets increases. For example, the condition includes the case that the sheet count of the fixing per unit time (namely, productivity) is large and the case that the weight of the recording material per unit area is large.
When the non-sheet passing portion temperature rise occurs due to continuous passing of the small size sheets, the temperature of the heating member exceeds the allowable range for usage. Consequently, the lifetime of the heating member is shortened.
Accordingly, a fan is disposed for blowing air to the heating member in Japanese Patent Application Laid-Open No. 2002-287564. Further, in Japanese Patent Application Laid-Open No. 2001-183929, a plurality of heaters whose heat generating amounts in the longitudinal direction differ from each other are disposed and the powering ratio of the heaters is changed in accordance with the size of the recording material.
With the configuration using a fan, there is a problem of noise caused by the rotation of the fan. With the method for changing the powering ratio, it is difficult to suppress a temperature rise at the non-sheet passing portion when a plurality of the recording materials are continuously heated.
As a countermeasure for addressing the problems of the two methods, it is considered that only the method for changing the powering ratio is adopted at the beginning and the air blowing is started after the temperature at the non-sheet passing portion becomes high when a plurality of the recording materials are continuously heated. However, in this case, the temperature at the boundary between the sheet passing portion and the non-sheet passing portion becomes low after air blowing and poor heating occurs.